Semiconductor devices that can emit non-coherent or coherent light are known in the art. A number of publications on semiconductor based light emitters deals with Light Emitting Diodes (LEDs) or Microcavity LEDs or Microcavity Lasers or Vertical Cavity Surface Emitting Lasers. Examples of such publications are:                H. De Neve, J. Blondelle, R. Baets, P. Demeester, P. Van Daele, G. Borghs, IEEE Photon. Technol. Lett. 7 287 (1995);        E. F. Schubert, N. E. J. Hunt, R. J. Malik, M. Micovic, D. L. Miller, “Temperature and Modulation Characteristics of Resonant-Cavity Light-Emitting Diodes”, Journal of Lightwave Technology, 14 (7), 1721-1729 (1996);        T. Yamauchi and Y. Arakawa, Enhanced and inhibited spontaneous emission in GaAs/AlGaAs vertical microcavity lasers with two kinds of quantum wells. Appl. Phys. Lett. 58 (21), 2339 (1991);        T. J. de Lyon, J. M. Woodall, D. T. McInturff, R. J. S. Bates, J. A. Kash, P. D. Kirchner, and F. Cardone, “Doping concentration dependence of radiance and optical modulation bandwidth in carbon-doped Ga0.1 In0.49P/GaAs light-emitting diodes grown by gas source molecular beam epitaxy” Appl. Phys. Lett. 60 (3), 353-355 (1992);        D. G. Deppe, J. C. Campbell, R. Kuchibhotla, T. J. Rogers, B. G. Streetman, “Optically-coupled mirror-quantum well InGaAs-GaAs light emitting diode”, Electron. Lett. 26 (20), 1665 (1990);        M. Ettenberg, M. G. Harvey, D. R. Patterson, “Linear, High-Speed, High-Power Strained Quantum-Well LED's”, IEEE Photon. Technol. Lett. 4 (1), 27 (1992);        U.S. Pat. No. 5,089,860 Deppe, et. al. Feb. 18, 1992, “Quantum well device with control of spontaneous photon emission, and method of manufacturing same”.        
It is known in the art that the light emission from an electroluminescent device or from a light emitting semiconductor diode (a LED) is limited by the total interal reflection occurring at the interface between the semiconductor substrate wherein the device is fabricated and the surrounding medium. Mostly emission of the light to air, with refractive index of unity, is intended. The semiconductor typically has a refractive index ns of 3 to 4. GaAs, for example, has a refractive index ns=3.65. Snell's law determines that only photons arriving at the semiconductor-air interface with an angle smaller than a critical angle θc=arcsin (1/ns) can escape to the air. All other photons are totally reflected at the semiconductor-air interface, and therefore remain in the semiconductor substrate, until eventually they are re-absorbed. For GaAs, the critical angle for total internal reflection is 16 degrees. Hence, total internal reflection limits the number of photons escaping the semiconductor substrate to those photons arriving at the semiconductor-air interface with an angle of less than 16 degrees. Only about 2% of the photons generated inside the semiconductor comply with this condition.
Several prior-art inventions propose to increase the escape probability of photons generated in the LED. In microcavity light-emitting diodes, such as described for example by Cho et al. in U.S. Pat. No. 5,226,053, the active layer of the light emitting device is placed in a microcavity. The cavity influences the emission of the photons: more photons are generated with an angle smaller than the critical angle θc. In this way, efficiencies of 15% and more have been achieved.
A second way to increase the efficiency of LEDs is to re-absorb photons which cannot escape from the semiconductor. If re-absorption occurs in the active layer of the LED, there is a chance that the electron-hole pair generated during re-absorption will recombine radiatively again, and re-emit a photon. Again, 2% of these photons will escape, and the remainder part can be re-absorbed. The phenomenon of multiple re-absorption and re-emission has been shown to result in efficiencies of the order of 10% in normal LEDs, and also to boost the efficiency of certain microcavity LEDs up to 23%. The problem with this technique is that it is inherently slow, because one has to wait for multiple re-absorptions and re-emissions.
A third way is to shape the semiconductor surface of the light emitting devices such that more of the generated rays reach the semiconductor-air interface within the critical angle. The optimum shape for the semiconductor-air surface is a hemisphere, where the light-emitting area is confined to a small spot at the centre of the hypothetical full sphere from which the hemisphere is taken. Other shapes have been proposed. In U.S. Pat. No. 5,087,949, Haitz proposes a structure which is more practical to make than a hemisphere, namely a set of V-groves in the substrate that are created such that the normal to the V-grove facets are oriented substantially perpendicularly to the light-emitting region. Kato, in U.S. Pat. No. 5,349,211, proposes a structure where the sidewalls or edges of the substrate are shaped such that some of the photons that are reflected from the regular light-output interface are emitted through these sidewalls. Egalon and Rogowski propose a sidewall shape for the substrate (rather than only for the mesa) that redirects some of the photons to angles that can escape through the regular light-output surface. All these proposed structures assume that the LED substrate is fairly thick and transparent for the photons emitted by the diode.
According to the teaching of U.S. Pat. No. 5,087,949 by Yamanaka et al. light emitting devices with a cavity having a truncated polyhedral pyramid shape are created. The lateral edges or facets of the devices have an angle of preferentially 45 degrees. Photons that are generated in a direction parallel to the light-output surface (edge) are reflected by the mesa edge of such cavity into a direction which is substantially perpendicular to the light-output surface, and hence more photons can escape from the cavity.
A fourth way to increase LED efficiency is to provide device structures capable to redirect a photon more than once before the photon is re-absorbed. This goal is achieved by providing surface edges of the device that contain surface portions having angles different from the main semiconductorsubstrate-air interface. Every time a photon hits such a surface, it is redirected to a new propagation angle. In this way, photons travelling in a direction that is not favourable for emission to the air have a certain probability to be redirected in a favourable angle after a number of reflections at such surface. In U.S. Pat. No. 3,739,217, Bergh and Saul propose to create topographic irregularities at the light-emitting surface or at the opposite (light-reflecting) surface of a LED which has a transparent substrate. Noguchi et al. in EP-A-0404565 propose to texture the sidewalls or edges of the substrate wherein the light emitting device is made.
The previously described methods apply to light-emitting devices where the light is emitted through the substrate. Therefore, the light-emitting devices need to be fabricated in a transparent substrate. The teaching of the patents of Noguchi et al., Egalon et al., and Kato et al. can only be applied to single light-emitting devices, but not to light-emitting devices in arrays. The invention disclosed by Haitz et al. (U.S. Pat. No. 5,087,949) can be applied to arrays, but it requires the presence of a fairly thick substrate, and the spacing of the light-emitting devices of an array should be of the order of the substrate thickness.
A method for manufacturing light-emitting devices the substrate of which does not have to be transparent is proposed in U.S. Pat. No. 5,358,880, by Lebby et al. The invention includes replacing the original substrate, which can be non-transparent, by a transparent conductive layer such as Indium Tin Oxide and a clear epoxy plus glass-like host substrate. Further, the disclosed invention aims to make a closed cavity, by etching through the active layer of the light emitting device, and covering the sides of the etched mesa with a dielectric material and a metal. In this way, photons that cannot escape through the transparent window are kept in the cavity by multiple reflections, re-absorbed and eventually re-emitted.
When applied to for instance a material system like GaAs, InAs, or AlAs and combinations thereof, the inventions mentioned above can only be used for light-emitting devices with a large diameter. This is because these prior art inventions rely on etching through the active layer of the light-emitting device. When doing so in a material with a large surface recombination velocity like GaAs, one triggers a severe parasitic surface recombination current. Surface recombination is a phenomenon of two charge carriers of opposite type, an electron and a hole, recombining at a trap, e.g. at the surface of the material, without emitting light. The velocity of recombination is proportional to the number of available traps and to the density of charge carriers available for recombination. The surface of III-V semiconductors like GaAs has a large number of traps, about 1014 cm−2. Hence, the surface recombination velocity is very large. Thus there is a problem of making highly efficient light emitting devices as the recombination losses of the charge carriers decrease the efficiency of the total light emission of the devices. One solution is to inject electrons and holes from electrodes that are physically at a large distance from the free surface (edge), which usually coincides with the cleavage surface of the device. Because of the necessity of having a large distance between the edge of the device and the contact, the resulting device is necessarily large, typically at least several hundred microns in diameter. Such devices therefore are not suitable for integration in large arrays. Also, their large area results in a large capacitance, and therefore slow operation.
Thus the prior art fails to disclose highly efficient light-emitting devices that can be integrated as small devices in a dense array of light emitting devices.
High efficiency LEDs are required for applications such as optical communication. Optical communication replaces electrical communication in many areas, because it can provide longer distance interconnects for a given energy budget. The minimum distance over which optical interconnects are advantageous over electrical interconnects can be quite short, e.g. a few centimeters. The energy consumption necessary for a given data bandwidth is a critical factor deciding on the minimum distance over which optical interconnects are competitive over electrical interconnects. The bandwidth transmitted by an optical interconnect system is the product of the serial bandwidth per interconnect channel with the number of parallel optical channels. Optics have the advantage over electrical interconnects that the number of channels can be much larger. One of the conditions to be able to access this potential massive parallelism, is that the power consumption per channel remains small: the heat dissipation has to remain manageable. Therefore high efficiency LEDs that can be integrated in dense arrays are needed.